The phenomenon of the “buckling” of the rotor and of the casing of turbomachines such as gas turbines and steam turbines is sufficiently known. It is caused by the large and high-mass structures of such machines having stored large quantities of heat after prolonged operation. During the cooling, a pronounced vertical thermal stratification occurs in the comparatively large flow passages, this thermal stratification leading to nonuniform temperature distributions in the static and the rotating components, which, on account of the different thermal expansions, results in a distortion of casing and rotor and in deviations from the rotationally symmetrical desired geometry. As a result, with the unavoidably small gap dimensions in modern turbomachines, jamming of the rotor in the casing occurs, which inhibits the start-up availability and in addition is capable of putting the mechanical integrity at risk. Shaft-turning systems or also “shaft-indexing” systems have therefore been disclosed, for example, by U.S. Pat. No. 3,793,905 or U.S. Pat. No. 4,854,120. In this case, the rotor of a turbomachine continues to be rotated at a certain speed after the shutdown. Here, as in the known shaft indexing, low speeds within the region of 1 rev/min and lower are preferred. On the one hand, this is sufficient in order to make the cooling of the rotor more uniform in the circumferential direction; on the other hand, the speed is low enough in order not to cause any pronounced axial flow through, for instance, the hot-gas path of a gas turbine with associated cold-air input and thermal shocks.
In the section subjected to high temperatures, modern gas turbines are often constructed with twin-shell casings. In this case, an annular space, to which cooling air or other coolant is often admitted during operation, is formed between an inner casing and an outer casing. A vertical thermal stratification which leads to distortion of the casings forms in the annular space without further measures after the gas turbine has been shut down.
DE 507 129 and WO 00/11324 propose to provide means in a twin-shell casing of a turbine in order to disturb the stable thermal stratification by a forced flow inside the intermediate space. In this case, it is essentially proposed to deliver fluid outside the annular space from one point of the annular space to another point of the annular space, as a result of which a compensating flow is induced inside the annular space. As assistance in this case, it is proposed to deliver comparatively cool fluid from the casing bottom part into the casing top part, or vice versa, in order to additionally utilize the differences in density for driving the convection flow. It may be stated that in DE 507 129 and WO 00/11324 the drive forces for the flow only act indirectly, as mentioned, in the form of the compensating flow. Furthermore, DE 507 129 and also WO 00/11324 do not suggest how the flow can be specifically set in a directional manner.